This invention relates to incandescent lamps in general and has particular reference to high power tungsten-halogen lamps and the filament mount structure thereof.
Tungsten-halogen lamps have become increasingly useful in lighting applications because of their higher efficiency, better maintenance and smaller size in comparison to prior art incandescent lamps. As a result of this commercial success, tungsten-halogen lamps are being developed with higher power ratings. These lamps are necessarily larger and present some problems that are not usually encountered with prior art incandescent lamps or lower wattage tungsten-halogen lamps. One problem concerns the adequacy of the support of the mount at the end opposite the lamp base, the seal of which is sometimes the only support for the mount. U.S. Pat. 3,497,752 shows a mount for such lamps. However, in that patent, the filament is supported only at the ends thereof, and thus can be supported directly by the tungsten lead-in wires.
A subsequent U.S. Pat. No. 3,543,962 concerns a folded filament which must be supported at other points thereof intermediate the filament legs and preferably at the filament folds. The fold support wires cannot be directly connected to the tungsten lead-in wires since connections would result in the shorting out of part of the filament.
Tungsten-halogen lamps having folded coiled filaments, as in the 3 962 patent, are sometimes preferable to those having coiled filaments, such as are disclosed in the '752 patent, where it is desired to reduce the effective area of incandescence of the filament as, for example, where the lamp is operated in conjunction with an external reflector and where efficient directional illumination is required, as in motion picture and television lighting.
In addition, the corrosive effect of halogen, especially at the high temperatures at which lamps of this type operate, severely limits the materials that can be used within a halogen lamp envelope. Generally, only tungsten and quartz are suitable. In lower wattage lamps, the filament can be welded or hot crimped to the lead-in wire. But, mainly because of the increased wire sizes required, such connections are not usually satisfactory in higher wattage lamps.
The above-mentioned U.S. Pat. No. 3,543,962 discloses a halogen lamp having a bridge which materially stabilizes the central positioning of a folded coiled filament mounted thereof. The bridge comprises quartz tubing supported by tungsten rods. Tungsten support wires, inserted through substantially diametral holes in the quartz tubing and supported thereby, insulatively support the filament at the folds thereof. The support wires have sufficient flexibility and adjustability to conveniently permit the takeup of any slack between the quartz tubing and the individual filament folds when the filament is mounted on the bridge. More specifically, one end of the support wire extends through a diametral hole in the quartz tubing and is bent therearound. The opposite end of the support wire extends through and engages a fold of the filament. Between its ends, the support wire has a substantially U-shaped loop, the loop having sufficient flexibility to permit the legs of the U to be squeezed together. Thus when the filament is mounted on the bridge, any slack in the filament can be taken up by squeezing the U-shaped loops of the support wires.
In some cases, it may be desirable to continue the end of the support wire that engages the filament fold back through the diametral hole in the quartz tubing. Thus, both ends of the support wire extend through the diametral hole and there is effectively a double wire supporting each filament fold. Such a construction can improve the resistance of the filament to sagging, especially when the lamp is operated in a horizontal position.
Preferably, the quartz tubing has slots at each end for the purpose of engaging the above-mentioned tungsten rods and to prevent rotation thereof, said rods also being the lead-in members for the lamp. Of course, the rods must have sufficient rigidity to permit take up of filament slack without deflection and to adequately support the filament mounted bridge. Preferably, also, electrical connection is established between the rod and the filament by a compressive connector coil of the type shown in U.S. Pat. No. 3,497,752. Said coil constrictively encircles the lead-in rod and an abutting portion of the filament leg. Protruding arms of these coils can provide some degree of mount stabilization, but for large high power lamps, the coil arms alone are not sufficient for this purpose and tend to break off.
Although the lamp described in U.S. Pat. No. 3,543,962 represented a significant improvement in the art, such a filament mount structure, when employed in large, high-power studio lamps, can exhibit fracture and breakage problems under shock and vibration. Previous approaches for countering this problem included projection of one of the supporting lead-in wires or rods (or a bridge-connected wire) into the exhaust tip to provide rigidity at the free end of the mount. This design was tried without success in large high power lamps, as, during shock and vibration testing, breakage occurred on the bridge member used to anchor the projecting wire. Other prior designs deal with the mount wobble problem by embedding the support rods at each end of the bulb in a press seal. Such a tungsten-to-quartz seal, however, can lead to cracks at the seal area due to thermal expansion differences, and the approach is somewhat unfeasible for large high-power lamps. Yet another prior method that has been employed to rigidize the mount of smaller low power lamps comprises the use of a wall bumper loop at the free ends of the tungsten support rod which bears against the dome-side area of the lamp envelope. For example, see U.S. Pat. No. 3,898,505. Upon examining this approach, however, we find such a construction to be relatively expensive and, in large high power lamps, the brittle tungsten loop is subject to breakage.
Still a further approach is shown in U.S. Pat. No. 3,626,236, wherein internally projecting quartz tubes are sealed to the interior of the envelope, and coils on the upper ends of the support rods fit into these tubes to secure the top of the mount. Although perhaps satisfactory in smaller, lower power lamps with a press seal base, such an upper support means is unsatisfactory for large high power lamp structures. The lever action of the rods or coils on the depending tubes can cause the tubes to break off, especially for the cup seal-supported rod structures typical for high power lamps. Further, the construction appears difficult and expensive to fabricate in production.
In addition to the above, quartz bridges have been found to be subject to breakage during mounting of the coil filament and under shock and vibration.